Combined space-time decoding

ABSTRACT

The present invention relates to decoding space time coded (STC) signals transmitted from a number of transmit antennas. First, a separate detection technique is used to determine initial decoding solutions corresponding to the symbols transmitted from each of a number of transmit antennas at a given time. For each initial solution, a limited area about the initial solution is defined. Each of the limited areas will correspond to regions including constellation points proximate the initial solution. The initial solutions are used to define a limited, multi-dimensional space. Accordingly, the initial solutions are used to reduce the search complexity associated with joint decoding by defining a limited space about the initial solutions. Finally, a joint decoding technique is implemented within the limited space to find a final solution.

FIELD OF THE INVENTION

The present invention relates to wireless communications, and inparticular to adaptively controlling coding and modulation techniques ina wireless communication system incorporating space-time coding.

BACKGROUND OF THE INVENTION

Interference and fading are significant impediments to achieving highdata rates in today's wireless communication systems. Given the tendencyfor channel conditions to sporadically and significantly fade,communication resources are conservatively allocated, leaving excessiveamounts of communication resources unused most of the time. Efforts tocombat the impact of fading include incorporating transmission diversityor controlling modulation and coding techniques in relation to channelconditions.

Spatial diversity is typically a function of the number and placement oftransmit and receive antennas relative to a transmitter and receiver.Systems employing spatial diversity with multiple transmit and receiveantennas are generally referred to as multiple-input multiple-output(MIMO) systems. Accordingly, a transmitting device will have M transmitantennas, and the receiving device will have N receive antennas.Space-time coding controls what data is transmitted from each of the Mtransmit antennas. A space-time encoding function at the transmitterprocesses data to be transmitted and creates unique information totransmit from the M transmit antennas. Each of the N receive antennaswill receive signals transmitted from each of the M transmit antennas. Aspace-time decoding function at the receiving device will combine theinformation sent from the M transmit antennas to recover the data.

Space-time coding (STC) is typically implemented using one of twotechniques. The first technique encodes the same data in differentformats for transmission from the different transmit antennas. Thus, thesame data is transmitted in different formats from each of the Mtransmit antennas. The second technique transmits different data fromdifferent ones of the M transmit antennas wherein the redundancy of thesecond technique is avoided. The first technique, space-time transmitdiversity (STTD), is effective in maximizing diversity but inefficientdue to the requisite redundancy. The second technique, which is oftenreferred to as V-BLAST (Vertical-Bell Laboratories Layered Space Time),increases system throughput for systems having sufficient diversityavailable. Once a threshold amount of diversity is achieved, data ratesincrease linearly with the number of transmit and receive antennas forBLAST systems, whereas additional spatial diversity has little impact ondata rates in STTD systems. Further information related to STTD andBLAST can be found in Siavash M. Alamouti, “A Simple Transmit DiversityTechnique for Wireless Communications,” IEEE J. Select. Areas Commun.,vol. 16, pp. 1451-1458, October 1998; G. J. Foschini, “LayeredSpace-time Architecture for Wireless Communications in a FadingEnvironment when Using Multi-element antennas,” Bell Labs Tech. J., pp.41-59, Autumn 1996; G. D. Golden, G. J. Foschini, R. A. Valenzuela, andP. W. Wolniansky, “Detection Algorithm and Initial Laboratory ResultsUsing V-BLAST Space-time Communication Architecture,” ElectronicsLetters, vol. 35, pp. 14-16, January 1999; and P. W. Wolniansky, G. J.,Foschini, G. D. Golden, and R. A. Valenzuela, “V-BLAST: An Architecturefor Realizing Very High Data Rates Over the Rich-scattering WirelessChannel,” Proc. IEEE ISSSE-98, Pisa, Italy, September 1998, pp. 295-300,which are incorporated herein by reference.

Prior to implementing space-time coding (STC), data to be transmitted isnormally encoded to facilitate error correction, and modulated or mappedinto symbols using any number of available modulation techniques, suchas quadrature phase shift keying (QPSK) and x-quadrature amplitudemodulation (QAM). The type of encoding for error correction andmodulation techniques greatly influences the data rates, and theirapplicability is often a function of channel conditions.

In general, BLAST-type STC decoding techniques are defined as providingeither joint or separate detection. For joint detection, to detect asignal from one antenna, the signals transmitted by all of the otherantennas must also be considered. In essence, the signals are detectedas a set, not on an individual basis as is done with separate detectiontechniques. The optimum algorithm to facilitate BLAST decoding ismaximum likelihood decoding (MLD), which is a joint detection technique.MLD obtains a diversity order equal to the number of receive antennas,independent of the number of transmit antennas. Hence, compared to othertechniques, MLD has a significant signal-to-noise ratio (SNR) advantage,and the SNR gain grows with the number of transmit antennas. Onedisadvantage of MLD is that its complexity grows exponentially with thenumber of transmit antennas. As such, the predominant decodingtechniques for BLAST implement separate detection using minimum meansquare error (MMSE) or zero-forcing (ZF), which are both significantlyless complex than MLD techniques.

In operation, a ZF V-BLAST decoder operates to invert the MIMO channeland solve the information transmitted from M transmit antennas. Supposethe MIMO channel input is denoted as vector:x=[x ₁ x ₂ . . . x _(M)]^(T),  Equation 1,the MIMO channel output is denoted as:y=[y ₁ y ₂ . . . y _(N)]^(T), and  Equation 2,the MIMO channel noise is denoted as:n=[n ₁ n ₂ . . . n _(N)]^(T).  Equation 3,In the presence of noise, the V-BLAST decoder input can be representedas:y=H _(N×M) x+n.  Equation 4,The V-BLAST decoder performs MIMO channel estimation H, then the directMIMO channel inversion leads to a zero-forcing solution, which isrepresented as:

$\begin{matrix}\begin{matrix}{\hat{x} = {\left( {H^{\prime}H} \right)^{- 1}H^{\prime}y}} \\{{= {x + ɛ}},}\end{matrix} & {{Equation}\mspace{14mu} 5}\end{matrix}$where {circumflex over (x)} is the detected signal vector of x, andε=−(H′H)⁻¹ H′n is the detection noise. Notably, in the derivation ofEquation 5, perfect channel information is assumed; thus when n=0, ε=0.

When making hard-decision symbol estimates, the Euclidean distancesbetween each detected signal {{circumflex over (x)}_(l)|_(l=1, M)},which is represented as a point in a constellation, and all the possiblesignal candidates from a particular antenna, which are represented asseveral points in a constellation lattice, are calculated. Next, thecandidate signal with the minimum distance to the detected signal isdeemed to be the signal transmitted. Note that the detection process foreach antenna is unrelated to signals transmitted from the otherantennas. The same is true for soft-decision bit de-mapping. Forinstance, the sign and value of each soft bit de-mapped from the signaltransmitted by the antenna Tx_(i) are determined by {circumflex over(x)}_(l) only, and are unrelated to {{circumflex over (x)}_(j)|_(j≠l)}.

As noted above, MLD takes a joint detection approach. To detect whichsignal has been transmitted by antenna Tx_(i), the decoder must considersignals transmitted by all the other antennas. As such, the signals aredetected as a set. The maximum likelihood estimate of a signal vector xis given by:{tilde over (x)}=arg min∥r−Hx∥,  Equation 6,where x is the signal vector given in Equation 1, with x₁ ∈{s₁, s₂, . .. , s_(q)} and q=2^(n) ^(b) being the constellation size. As shown fromEquation 6, the detection is based on the Euclidean distance betweenreceived signal vector r and q^(M) different candidate vectors Hx, whereM is the number of transmit antennas.

The maximum likelihood estimate of a received signal vector is given byEquation 6, which expands the candidate signal constellation from onedimension to M dimensions. Accordingly, the complexity growsexponentially with the number of transmit antennas. With separatedetection, the Euclidean distance calculation is always carried out inone dimension only:{tilde over (x)} _(l) =arg min∥{circumflex over (x)} _(l) −x_(l)∥.  Equation 7

Accordingly, the computational complexity of V-BLAST detection forseparate detection techniques, such as ZF or MMSE, is linear withrespect to the number of transmit antennas. However, the limitation forthe separate detection techniques is twofold. First, the number ofreceiver antennas should be larger than the number of transmit antennas.Second, the diversity order with respect to the system performance isN−M+1. In contrast, the advantages of joint detection are that thediversity order for the MLD decoder is linear with the number oftransmit antennas, and the number of receive antennas can be less thanthe number of transmit antennas. However, the disadvantage of the jointdetection is that the complexity of the MLD decoder is exponential tothe number of transmit antennas. Thus, there is a need for a decodingtechnique capable of taking advantage of the benefits of both joint andseparate decoding while minimizing their limitations. In particular,there is a need for the precision of MLD detection with the complexitycloser to ZF or MMSE detection.

Further reference is made to Richard van Nee, Allert van Zelst and GeertAwater, “Maximum Likelihood Decoding in a Space Division MultiplexingSystem,” IEEE VTC 2000, Tokyo, Japan, May 2000, and Andrej Stefanov andTolga M. Duman, “Turbo-coded Modulation for Systems with Transmit andReceive Antenna Diversity over Block Fading Channels: System Model,Decoding Approaches, and Practical Considerations,” IEEE J. Select.Areas Commun., vol. 19, pp. 958-968, May, 2001, which are incorporatedherein by reference.

SUMMARY OF THE INVENTION

The present invention relates to decoding space time coded (STC) signalstransmitted from a number of transmit antennas. The decoding techniqueinvolves the following. First, a separate detection technique, such aszero-forcing (ZF) or minimum mean square error (MMSE), is used todetermine initial decoding solutions corresponding to the symbolstransmitted from each of a number of transmit antennas at a given time.For each initial solution, a limited area about the initial solution isdefined. Each of the limited areas will correspond to regions includingconstellation points proximate the initial solution. The initialsolutions are used to define a limited, multi-dimensional space.Accordingly, the initial solutions are used to reduce the searchcomplexity associated with joint decoding by defining a limited spaceabout the initial solutions. Finally, a joint decoding technique, suchas maximum likelihood decoding MLD, is implemented within the limitedspace to find a final solution. In a further refinement of the presentinvention, the initial solutions may be derived from signalscorresponding to only select transmit antennas, wherein successive MLDsolutions are used to find the final, MLD solution to provideinterference cancellation and reduce processing complexity.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a block representation of a wireless communication systemaccording to one embodiment of the present invention.

FIG. 2 is a block representation of a base station according to oneembodiment of the present invention.

FIG. 3 is a block representation of a mobile terminal according to oneembodiment of the present invention.

FIG. 4 is a logical breakdown of a transmitter architecture according toone embodiment of the present invention.

FIG. 5 is a logical breakdown of a receiver architecture according toone embodiment of the present invention.

FIGS. 6A and 6B are constellation lattices from a transmitter andreceiver's perspective and illustrate operation of the presentinvention.

FIG. 7 is a flow diagram illustrating operation of one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

The present invention provides a space-time coding (STC) technique forreceiving signals in a multiple-input multiple-output (MIMO) systemwhere joint decoding is provided in an efficient and less complex mannerthan previously available. Instead of relying solely on separatedecoding techniques, such as zero-forcing (ZF) or minimum mean squareerror (MMSE), or relying on joint decoding techniques, such as maximumlikelihood decoding (MLD), a combination of separate and joint decodingis implemented. In general, the decoding technique involves thefollowing. Initially, a separate detection technique is used todetermine initial decoding solutions corresponding to the symbolstransmitted from each of a number of transmit antennas at a given time.For each initial solution, a limited area about the initial solution isdefined. Each limited area will correspond to regions includingconstellation points proximate the initial solution. The initialsolutions are used to define a limited, multi-dimensional space.Accordingly, the initial solutions are used to reduce the searchcomplexity associated with joint decoding by defining a limited spaceabout the initial solutions. Finally, a joint decoding technique isimplemented within the limited space to find a final solution. Otherimprovements and details are described further below. A high leveloverview of mobile terminal and base station architectures of thepresent invention are provided prior to delving into the structural andfunctional details of the decoding techniques of the present invention.

With reference to FIG. 1, a basic wireless communication environment isillustrated. In general, a base station controller (BSC) 10 controlswireless communications within multiple cells 12, which are served bycorresponding base stations (BS) 14. Each base station 14 facilitatescommunications with mobile terminals 16, which are within the cell 12associated with the corresponding base station 14. For the presentinvention, the base stations 14 and mobile terminals 16 include multipleantennas to provide spatial diversity for communications.

With reference to FIG. 2, a base station 14 configured according to oneembodiment of the present invention is illustrated. The base station 14generally includes a control system 20, a baseband processor 22,transmit circuitry 24, receive circuitry 26, multiple antennas 28, and anetwork interface 30. The receive circuitry 26 receives radio frequencysignals through antennas 28 bearing information from one or more remotetransmitters provided by mobile terminals 16. Preferably, a low noiseamplifier and a filter (not shown) cooperate to amplify and removebroadband interference from the signal for processing. Downconversionand digitization circuitry (not shown) will then downconvert thefiltered, received signal to an intermediate or baseband frequencysignal, which is then digitized into one or more digital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs). Thereceived information is then sent across a wireless network via thenetwork interface 30 or transmitted to another mobile terminal 16serviced by the base station 14. The network interface 30 will typicallyinteract with the base station controller 10 and a circuit-switchednetwork forming a part of a wireless network, which may be coupled tothe public switched telephone network (PSTN).

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by a carrier signal having a desiredtransmit frequency or frequencies. A power amplifier (not shown) willamplify the modulated carrier signal to a level appropriate fortransmission, and deliver the modulated carrier signal to the antennas28 through a matching network (not shown). The multiple antennas 28 andthe replicated transmit and receive circuitries 24, 26 provide spatialdiversity. Modulation and processing details are described in greaterdetail below.

With reference to FIG. 3, a mobile terminal 16 configured according toone embodiment of the present invention is illustrated. Similarly to thebase station 14, the mobile terminal 16 will include a control system32, a baseband processor 34, transmit circuitry 36, receive circuitry38, multiple antennas 40, and user interface circuitry 42. The receivecircuitry 38 receives radio frequency signals through antennas 40bearing information from one or more base stations 14. Preferably, a lownoise amplifier and a filter (not shown) cooperate to amplify and removebroadband interference from the signal for processing. Downconversionand digitization circuitry (not shown) will then downconvert thefiltered, received signal to an intermediate or baseband frequencysignal, which is then digitized into one or more digital streams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations, as will be discussed on greater detail below. Thebaseband processor 34 is generally implemented in one or more digitalsignal processors (DSPs) and application specific integrated circuits(ASICs).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, data, or control information, from thecontrol system 32, which it encodes for transmission. The encoded datais output to the transmit circuitry 36, where it is used by a modulatorto modulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). The multiple antennas 40 and the replicated transmit andreceive circuitries 36, 38 provide spatial diversity. Modulation andprocessing details are described in greater detail below.

With reference to FIG. 4, a logical transmission architecture isprovided according to one embodiment. The transmission architecture isdescribed as being that of the base station 14, but those skilled in theart will recognize the applicability of the illustrated architecture forboth uplink and downlink communications. Further, the transmissionarchitecture is intended to represent a variety of multiple accessarchitectures, including, but not limited to code division multipleaccess (CDMA), frequency division multiple access (FDMA), time divisionmultiple access (TDMA), and orthogonal frequency division multiplexing(OFDM).

Initially, the base station controller 10 sends data 44 intended for amobile terminal 16 to the base station 14 for scheduling. The scheduleddata 44, which is a stream of bits, is scrambled in a manner reducingthe peak-to-average power ratio associated with the data using datascrambling logic 46. A cyclic redundancy check (CRC) for the scrambleddata is determined and appended to the scrambled data using CRC logic48. Next, channel coding is performed using channel encoder logic 50 toeffectively add redundancy to the data to facilitate recovery and errorcorrection at the mobile terminal 16. The channel encoder logic 50 usesknown Turbo encoding techniques in one embodiment. The encoded data isthen processed by rate matching logic 52 to compensate for the dataexpansion associated with encoding.

Bit interleaver logic 54 systematically reorders the bits in the encodeddata to minimize the loss of consecutive data bits. The resultant databits are systematically mapped into corresponding symbols depending onthe chosen baseband modulation by mapping logic 56. Preferably, a formof Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key(QPSK) modulation is used. The symbols may be systematically reorderedto further bolster the immunity of the transmitted signal to periodicdata loss caused by frequency selective fading using symbol interleaverlogic 58.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. Blocks of symbols arethen processed by space-time code (STC) encoder logic 60. The STCencoder logic 60 will process the incoming symbols according to aselected STC encoding mode and provide n outputs corresponding to thenumber of transmit antennas 28 for the base station 14. Further detailregarding the STC encoding is provided later in the description. At thispoint, assume the symbols for the n outputs are representative of thedata to be transmitted and capable of being recovered by the mobileterminal 16. Further detail is provided in A. F. Naguib, N. Seshadri,and A. R. Calderbank, “Applications of space-time codes and interferencesuppression for high capacity and high data rate wireless systems,”Thirty-Second Asilomar Conference on Signals, Systems & Computers,Volume 2, pp. 1803-1810, 1998; R. van Nee, A. van Zelst and G. A.Atwater, “Maximum Likelihood Decoding in a Space Division MultiplexSystem”, IEEE VTC. 2000, pp. 6-10, Tokyo, Japan, May 2000; and P. W.Wolniansky et al., “V-BLAST: An Architecture for Realizing Very HighData Rates over the Rich-Scattering Wireless Channel,” Proc. IEEEISSSE-98, Pisa, Italy, Sep. 30, 1998 which are incorporated herein byreference in their entireties.

For illustration, assume the base station 14 has two antennas 28 (n=2)and the STC encoder logic 60 provides two output streams of symbols.Accordingly, each of the symbol streams output by the STC encoder logic60 is sent to a corresponding multiple access modulation function 62,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch analog or digital signal processing alone or in combination withother processing described herein. For example, the multiple accessmodulation function 62 in a CDMA function would provide the requisite PNcode multiplication, wherein an OFDM function would operate on therespective symbols using inverse discrete Fourier transform (IDFT) orlike processing to effect an Inverse Fourier Transform. Attention isdrawn to co-assigned application Ser. No. 10/104,399, filed Mar. 22,2002, entitled SOFT HANDOFF FOR OFDM, for additional OFDM details, andto RF Microelectronics by Behzad Razavi, 1998 for CDMA and othermultiple access technologies, both of which are incorporated herein byreference in their entirety.

Each of the resultant signals is up-converted in the digital domain toan intermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) circuitry 64 anddigital-to-analog (D/A) conversion circuitry 66. The resultant analogsignals are then simultaneously modulated at the desired RF frequency,amplified, and transmitted via the RF circuitry 68 and antennas 28.Notably, the transmitted data may be preceded by pilot signals, whichare known by the intended mobile terminal 16. The mobile terminal 16,which is discussed in detail below, may use the pilot signals forchannel estimation and interference suppression and the header foridentification of the base station 14.

Reference is now made to FIG. 5 to illustrate reception of thetransmitted signals by a mobile terminal 16. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal16, the respective signals are demodulated and amplified bycorresponding RF circuitry 74. For the sake of conciseness and clarity,only one of the multiple receive paths in the receiver is described andillustrated in detail. Analog-to-digital (A/D) conversion anddownconversion circuitry (DCC) 76 digitizes and downconverts the analogsignal for digital processing. The resultant digitized signal may beused by automatic gain control circuitry (AGC) 78 to control the gain ofthe amplifiers in the RF circuitry 74 based on the received signallevel.

The digitized signal is also fed to synchronization circuitry 80 and amultiple access demodulation function 82, which will recover theincoming signal received at a corresponding antenna 40 at each receiverpath. The synchronization circuitry 80 facilitates alignment orcorrelation of the incoming signal with the multiple access demodulationfunction 82 to aid recovery of the incoming signal, which is provided toa signaling processing function 84 and channel estimation function 86.The signal processing function 84 processes basic signaling and headerinformation to provide information sufficient to generate a channelquality measurement, which may bear on an overall signal-to-noise ratiofor the link, which takes into account channel conditions and/orsignal-to-noise ratios for each receive path.

The channel estimation function 86 for each receive path provideschannel responses corresponding to channel conditions for use by an STCdecoder 88. The symbols from the incoming signal and channel estimatesfor each receive path are provided to the STC decoder 88, which providesSTC decoding on each receive path to recover the transmitted symbols.The channel estimates provide sufficient channel response information toallow the STC decoder 88 to decode the symbols according to the STCencoding used by the base station 14.

The recovered symbols are placed back in order using the symbolde-interleaver logic 90, which corresponds to the symbol interleaverlogic 58 of the base station 14. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 92. The bits are then de-interleaved using bit de-interleaverlogic 94, which corresponds to the bit interleaver logic 54 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 96 and presented to channel decoder logic 98 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 100 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 102 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data 104.

A channel quality indicator (CQI) may be determined based on therecovered data. An additional or alternative CQI function 106 may beprovided anywhere along the data recovery path (blocks 90 through 104)to monitor signal-to-noise ratios, error rates, and like to deriveinformation bearing on individual or overall channel quality. Additionalinformation on one exemplary way to determine a CQI value is provided inco-assigned application Ser. No., 60/329,511, filed Oct. 17, 2001, andentitled “METHOD AND APPARATUS FOR CHANNEL QUALITY MEASUREMENT FORADAPTIVE MODULATION AND CODING.”

The following describes the overall functionality of the presentinvention and refers to the primary device used for transmission as thetransmitter and the device used for receiving as the receiver. At anygiven time depending on the direction of primary communications, thebase station 14 and the mobile terminals 16 may be a transmitter,receiver, or both.

The present invention reduces the complexity associated with MLDdecoding with respect to the transmit modulation by setting it to afixed value, which is invariant to the QAM size. Further, the MLDdecoding complexity becomes linear, instead of exponential, to thenumber of transmit antennas. In general, the decoding technique involvesthe following. First, a separate detection technique, such as ZF orMMSE, is used to determine initial decoding solutions corresponding tothe symbols transmitted from each of a number of transmit antennas at agiven time. For each initial solution, a limited area about the initialsolution is defined. Each of the limited areas will correspond toregions including constellation points proximate the initial solution.The initial solutions are used to define a limited, multi-dimensionalspace. Accordingly, the initial solutions are used to reduce the searchcomplexity associated with joint decoding by defining a limited spaceabout the initial solutions. Finally, a joint decoding technique, suchas MLD, is implemented within the limited space to find a finalsolution. In a further refinement of the present invention, the initialsolutions may be derived from signals corresponding to only selecttransmit antennas, wherein successive MLD solutions are used to find thefinal, MLD solution to provide interference cancellation and reduceprocessing complexity.

The basic concepts of a fast V-BLAST MLD decoder can be illustrated inFIGS. 6A and 6B. For the purpose of illustration, assume the transmitmodulation is 16 QAM, and the MIMO configuration is two transmitantennas (M=2) and four receive antennas (N=4). Accordingly, there are16 constellation points for basic 16 QAM modulation. Since there are twotransmit antennas, the total possible points at the receiver isrepresented by Q^(M), where Q is the modulation order and M is thenumber of transmit antennas. In this example, the total number ofpossible points at the receiver is 256 (16²).

With particular reference to FIG. 6A, constellations from theperspective of each of the two transmit antennas are illustrated. Eachconstellation has 16 possible points, wherein the circled pointrepresents the actual symbol being transmitted from the particularantenna. For reference, a ZF solution associated with the transmittedsymbol is mapped onto the transmission constellation to illustrate theeffect of the channel conditions on the transmitted symbol. Asillustrated, the channel conditions may impact the transmitted symbolsufficiently, where the ZF solution may not be most proximate to theactual constellation point corresponding to the symbol transmitted.

Accordingly, the ZF function is used to map the received symbol back toa point on the transmit constellation lattice for each receive antenna.This provides the initial solutions corresponding to symbols transmittedfrom each transmit antenna. The initial solutions act as rough estimatesof the received symbols. The initial solutions are then used to identifyor determine search areas, which are smaller regions within the transmitconstellation matrix, highlighted with dashed boxes and associated witha limited set of candidate constellation points. The smaller regionsdetermined based on the initial solutions define a limited,multi-dimensional space in which to implement MLD decoding. The limitedspace from the perspective of each receive antenna is illustrated inFIG. 6B. Within the limited space illustrated for each of the fourreceive antennas, there are 16 points, which correspond to the fourpoints within the limited space from each of the two transmit antennas.Thus, the number of points illustrated in the limited space is 4², or16. The asterisk represents the combination of symbols transmitted fromthe two transmit antennas as received at each of the four receiveantennas. The circled constellation point represents the combination ofthe transmitted symbols.

As noted, MLD decoding is provided only for the limited space determinedbased on the ZF solutions, instead of throughout the entireconstellation lattice as provided in FIG. 6A. The result of the MLDdecoding will provide hard-decisions for the symbols transmitted fromeach of the transmit antennas. In one embodiment, the initial solutionsare correlated with the nearest four constellation points based on theEuclidean distances. Using ZF or MMSE solutions to reduce the MLDdecoding search set significantly reduces processing complexity byreducing the space in which MLD decoding would otherwise be required.The limited space determined based on the initial ZF solutions maycorrespond to any number of possible points in the constellation latticeor area therein. Accordingly, the limited area for MLD decoding can besignificantly reduced, thus reducing the complexity of MLD. In theexample provided, the present invention reduces the required MLD from anoriginal constellation of 16 points to one of four points within thelarger constellation (as shown in FIG. 6A). Thus, the number of pointsfor MLD processing is reduced in a two-transmitter system from 16²=256to 4²=16 points. Tests have shown that limiting MLD decoding to such areduced set of constellation points as small as four can be done withoutperformance loss.

A process flow is next described for carrying out an enhanced embodimentof the present invention wherein MLD decoding is iteratively carried outfor groups of transmit antennas instead of for every transmit antenna atone time. Although the groups can be any size, the exemplaryimplementation operates on groups of two antennas. In this example,assume that there are four transmit antennas. In general, initial (ZF orMMSE) solutions are determined for each transmit antenna and used togroup the transmit antennas into groups based on signal strength. Inthis example of four transmit antennas, there are two groups of twoantennas. Next, the initial solutions for the weaker group aresubtracted from the stronger group to form modified initial solutions.For each modified initial solution, a limited area about the modifiedinitial solution is defined. Each of the limited areas will correspondto regions including constellation points proximate the modified initialsolution. The initial solutions are used to define a limited,multi-dimensional space. A joint decoding technique, such as MLD, isimplemented within the limited space to find a final solution for thestrongest pair of transmit antennas. These final solutions are thenmultiplied by the corresponding channel matrix to provide a modifiedsignal, which is subtracted from the received signal. The resultingsignal should represent the signals transmitted from the next, and final(or weaker) in the example, pair of transmit antennas.

A more detailed process flow for the latter embodiment is illustrated inFIG. 7. Accordingly, the channel matrix H is first determined (block200). Next, the inverse of the channel matrix is calculated (block 202)to determine the ZF (or MMSE) initial solutions, preferably using theMoore-Penrose pseudoinverse of the channel matrix (block 204). The orderin which to apply MLD decoding can be determined by sorting the norm ofcolumns of the pseudoinverse of the channel matrix H (block 206). Inthis embodiment, the ordering is used to group the transmit antennasinto pairs of antennas for MLD decoding (block 208), preferably based onsignal strength.

Next, MLD decoding is applied for each transmit antenna pair. Initially,a variable i is set equal to i+1 (block 210). If i is greater than thenumber of pairs of transmit antennas, i>M/2 (block 212), the process isstopped (block 214). Otherwise, a defined number, such as four, of theclosest or neighboring constellation points in the transmissionconstellation lattice to the ZF (or MMSE) function are identified forthe ith pair of antennas (block 216). MLD decoding is then performedabout the identified constellation points for the ith pair of transmitantennas to arrive at a final (MLD) solution (block 218). The final(MLD) solution in light of the channel conditions is then subtractedfrom the received signal to arrive at modified initial (ZF) solutionsfor the next pair of antennas (block 220). The process will repeat untila final (MLD) solution is provided for each pair of antennas. Notably,the group size may change from iteration to iteration. The advantages ofthe proposed approach follow. First, the MLD decoding complexity isindependent of QAM modulation size. In the above example, the MLDsearching range is reduced to the vicinity of four constellation points.Further, the MLD decoding is only applied to transmit antenna pairs andis a conditional MLD search based on the tentative solution ofzero-forcing.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

1. A method for decoding space time coded (STC) signals comprising: a)receiving STC signals originating from a plurality of transmit antennasof a remote transmitter; b) decoding the STC signals using a separateSTC decoding technique to determine a plurality of initial solutions; c)identifying a selected area about each of the initial solutions, whereinthe selected area is an area smaller than an entire constellationlattice; d) creating a decoding space corresponding to the selectedareas; and e) decoding the STC signals within the decoding space using ajoint STC decoding technique to determine a final solution representingsymbols transmitted from the plurality of transmit antennas.
 2. Themethod of claim 1 wherein the selected area corresponds to a limited setof constellation points in the constellation lattice containingconstellation points corresponding to possible symbols transmitted fromthe plurality of transmit antennas.
 3. The method of claim 1 wherein theselected area corresponds to a set of four constellation points in theconstellation lattice containing constellation points corresponding topossible symbols transmitted from the plurality of transmit antennas. 4.The method of claim 1 wherein the decoding space is a limited space in amulti-dimensional constellation lattice corresponding to a limited setof constellation points.
 5. The method of claim 1 wherein there is aninitial solution for each of the plurality of transmit antennas.
 6. Themethod of claim 1 wherein the separate STC decoding technique iszero-forcing.
 7. The method of claim 1 wherein the separate STC decodingtechnique is minimum mean square error decoding.
 8. The method of claim1 wherein the joint decoding technique is maximum likelihood decoding.9. The method of claim 1 wherein the STC signals are BLAST signals. 10.The method of claim 1, wherein the selected area is larger than aspecific constellation point.
 11. A method for decoding space time coded(STC) signals comprising: a) receiving STC signals originating from aplurality of transmit antennas of a remote transmitter; b) decoding theSTC signals using a separate STC decoding technique to determine aplurality of initial solutions; c) selecting a group of the initialsolutions associated with a group of the transmit antennas; d) removinginitial solutions outside of the selected group of the initial solutionsassociated with the group of the transmit antennas to provide modifiedinitial solutions for the group of the transmit antennas; e) identifyinga selected area about each of the modified initial solutions, whereinthe selected area is an area smaller than an entire constellationlattice; f) creating a decoding space corresponding to the selectedareas; and g) decoding the STC signals within the decoding space using ajoint STC decoding technique to determine a final solution representingsymbols transmitted from the transmit antennas associated with the groupof the transmit antennas.
 12. The method of claim 11 further comprising:h) recreating an interference cancellation signal based on the finalsolution; and i) subtracting the interference cancellation signal fromthe STC signals to create modified STC signals.
 13. The method of claim12 wherein the modified STC signals and step b through i are iterativelyrepeated until the final solutions are obtained for each group oftransmit antennas.
 14. The method of claim 11 wherein the selected areacorresponds to a limited set of constellation points in theconstellation lattice containing constellation points corresponding topossible symbols transmitted from the plurality of transmit antennas.15. The method of claim 11 wherein the selected area corresponds to aset of four constellation points in the constellation lattice containingconstellation points corresponding to possible symbols transmitted fromthe plurality of transmit antennas.
 16. The method of claim 11 whereinthe decoding space is a limited space in a multi-dimensionalconstellation lattice corresponding to a limited set of constellationpoints.
 17. The method of claim 11 wherein there is an initial solutionfor each of the plurality of transmit antennas.
 18. The method of claim11 wherein the separate STC decoding technique is zero-forcing.
 19. Themethod of claim 11 wherein the separate STC decoding technique isminimum mean square error decoding.
 20. The method of claim 11 whereinthe joint decoding technique is maximum likelihood decoding.
 21. Anapparatus for receiving space time coded (STC) signals comprising: a)receiver circuitry for receiving and demodulating modulated STC signalsto provide STC signals; b) an STC decoder for: i) receiving the STCsignals originating from a plurality of transmit antennas of a remotetransmitter; ii) decoding the STC signals using a separate STC decodingtechnique to determine a plurality of initial solutions; iii)identifying a selected area about each of the initial solutions, whereinthe selected area is an area smaller than an entire constellationlattice; iv) creating a decoding space corresponding to the selectedareas; and v) decoding the STC signals within the decoding space using ajoint STC decoding technique to determine a final solution representingsymbols transmitted from the plurality of transmit antennas.
 22. Theapparatus of claim 21 wherein the selected area corresponds to a limitedset of constellation points in the constellation lattice containingconstellation points corresponding to possible symbols transmitted fromthe plurality of transmit antennas.
 23. The apparatus of claim 21wherein the selected area corresponds to a set of four constellationpoints in the constellation lattice containing constellation pointscorresponding to possible symbols transmitted from the plurality oftransmit antennas.
 24. The apparatus of claim 21 wherein the decodingspace is a limited space in a multi-dimensional constellation latticecorresponding to a limited set of constellation points.
 25. Theapparatus of claim 21 wherein there is an initial solution for each ofthe plurality of transmit antennas.
 26. The apparatus of claim 21wherein the separate STC decoding technique is zero-forcing.
 27. Theapparatus of claim 21 wherein the separate STC decoding technique isminimum mean square error decoding.
 28. The apparatus of claim 21wherein the joint decoding technique is maximum likelihood decoding. 29.The apparatus of claim 21 wherein the STC signals are BLAST signals. 30.An apparatus for receiving space time coded (STC) signals comprising: a)receiver circuitry for receiving and demodulating modulated STC signalsto provide STC signals; b) an STC decoder for: i) receiving the STCsignals originating from a plurality of transmit antennas of a remotetransmitter; ii) decoding the STC signals using a separate STC decodingtechnique to determine a plurality of initial solutions; iii) selectinga group of the initial solutions associated with a group of the transmitantennas; iv) removing initial solutions outside of the selected groupof the initial solutions associated with the group of the transmitantennas to provide modified initial solutions for the group of thetransmit antennas; v) identifying a selected area about each of themodified initial solutions, wherein the selected area is an area smallerthan an entire constellation lattice; vi) creating a decoding spacecorresponding to the selected areas; and vii) decoding the STC signalswithin the decoding space using a joint STC decoding technique todetermine a final solution representing symbols transmitted from thetransmit antennas associated with the group of the transmit antennas.31. The apparatus of claim 30 wherein the STC decoder further: viii)recreates an interference cancellation signal based on the finalsolution; and ix) subtracts the interference cancellation signal fromthe STC signals to create modified STC signals.
 32. The apparatus ofclaim 31 wherein the modified STC signals become the STC signals andsteps ii) through ix) are iteratively repeated until final solutions areobtained for each group of transmit antennas.
 33. The apparatus of claim30 wherein the selected area corresponds to a limited set ofconstellation points in the constellation lattice containingconstellation points corresponding to possible symbols transmitted fromthe plurality of transmit antennas.
 34. The apparatus of claim 30wherein the selected area corresponds to a set of four constellationpoints in the constellation lattice containing constellation pointscorresponding to possible symbols transmitted from the plurality oftransmit antennas.
 35. The apparatus of claim 30 wherein the decodingspace is a limited space in a multi-dimensional constellation latticecorresponding to a limited set of constellation points.
 36. Theapparatus of claim 30 wherein there is an initial solution for each ofthe plurality of transmit antennas.
 37. The apparatus of claim 30wherein the separate STC decoding technique is zero-forcing.
 38. Theapparatus of claim 30 wherein the separate STC decoding technique isminimum mean square error decoding.
 39. The apparatus of claim 30wherein the joint decoding technique is maximum likelihood decoding.